Look at it another way. Shell/Texaco/Esso/BP and co spend billions developing and refining fuels, Tesco spend nothing, they simply buy the bog standard unwanted fuel from the big boys via a 3rd party company. Shel and co add cleaning agents to their fuel, which actively help keep your fuel system in good working order.
Imho, its worth going to the likes of Shell and BP, firstly it doesnt usually cost anymore (in fact the Tesco in Bidston is more often then not dearer then the Shell in Moreton) and secondly there are far too many stories of supermarket fuel being low grade (Many internet forums and independent reports suggested that Optimax at 98RON, was better then Tesco 99RON for both performance in imported Japanese cars and MPG overall).
I just stick with the big boys, the companies thats sole industry is Oil, they know what their doing.